This application is related to copending application U.S. Ser. No. 08/540,114, entitled "Mounting Structure for Combined Automotive Trim Accessory and Antenna", filed concurrently herewith and incorporated herein by reference.
The present invention relates in general to a concealed antenna for use on a vehicle and more specifically to a dual FM slot and AM monopole antenna that can be concealed within an exterior trim accessory such as a spoiler.
The most commonly used type of antenna for radio reception in automotive vehicle has been the standard whip antenna. The whip antenna has been desirable because of its good antenna performance in terms of antenna gain and directionality. Nevertheless, automotive manufacturers have sought alternatives to whip antennas because whip antennas are susceptible to damage (e.g., being bent or broken off), create wind noise, and are unattractive from a styling standpoint. However, concealed antenna designs which provide performance comparable to whip antennas and which may be made at a low cost have remained elusive.
The slot antenna is one type of concealed antenna that has been employed on automotive vehicles. A slot may be formed by a window aperture or by special composite materials used for body panels. Slot antennas, however, have not been well suited to reception in more than one frequency band. In slot antenna design, a slot is provided with a length about equal to one half the wavelength of the desired radio signals to be received. Thus, a single slot antenna is not well suited to receive both AM and FM radio signals because of the great difference between AM and FM wavelengths. Multiple slot antennas can be provided to obtain reception in multiple frequency bands, but a multiple slot design results in increase cost. Likewise, slot antenna designs employing body panels formed of composite materials are relatively expensive.
An on-glass antenna is another type of conformal (i.e., concealed) antenna. Antenna conductors are typically deposited on glass sheets in patterns that form separate FM and AM antennas. Typically, the AM antenna conductors also function as the heater grid for the glass window. However, in order to combine the AM and FM signal onto one transmission line to the radio receiver, special electronics are required to isolate the AM signals from the heater grid power voltage and to introduce the AM and FM signals to the coaxial transmission line. These special electronics typically require an additional electronic module and result in increased expense.